The cestode Schistocephalus solidus is a simultaneous hermaphrodite that reproduces in the gut of birds, or for this study in an in vitro system that simulates the gut of the bird. Like many other helminth parasites, S. solidus can reproduce by self- and cross-fertilization. Hermaphrodites are expected to mate not primarily to get their own eggs fertilized, but rather to get the opportunity to fertilize a partner's eggs. Because S. solidus has a size-dependent sex allocation (i.e., larger worms are more biased toward female allocation and produce more egg mass), we expect larger individuals to be attractive mating partners for smaller ones. However, this may be a one-directional preference, as smaller individuals may not be attractive to larger ones. We tested this experimentally by studying the reaction of focal worms of different sizes to a compartment containing a potential mating partner that was either smaller or larger than the focal worm. The focal worn-is were, on average, closer to the compartment containing the stimulus than to an empty control compartment. Moreover, they indeed showed a preference for larger stimulus worms than for smaller ones. They even tended to avoid being close to stimulus worms of very small size compared to themselves. This may reveal a general preference for cross-fertilization over selfing, but it also indicates that all the genetic benefits from outcrossing do not necessarily outweigh the costs of mating with a relatively small individual and that the worms may take this into account in their reproductive decisions.